Arch Question

[couchpotatoe]

OK, I've seen many, many arches in this forum. Would the one pictured below be just as strong - or stronger? Using a 9" x 4.5" firebrick, wouldn't it be like just leaving out the mortar with more smaller firebricks? Thanks for any insight

Russ

[Tscarborough]

It is very shallow and would require large buttresses.

[david s]

Originally Posted by Tscarborough

It is very shallow and would require large buttresses.

That's right. The shallower the arch (the larger the radius) then the greater is the sideways thrust. That is why the semi circular Roman arch is so strong and does not need much buttressing.

[couchpotatoe]

Thanks guys. That lets me breath easier because this will be a simple arch to make and should look good (although it will be behind a face brick arch). I was planning on placing very large buttresses, with thermal breaks made out of insulating firebrick, on either side of the vent area to contain the outward forces. I'm trying to make the arch high (10") enough to be usable for both bread and pizza baking but to stay with the golden 63%.

Russ

[GianniFocaccia]

Russ,

From your drawing, it appears your design has standard firebricks with their narrow side facing down. Two questions: If so, do you plan to support a vent/flue with this arch? Is your design made to simplify construction of the arch?

[couchpotatoe]

GianniFocaccia-
Yes, that was the plan. The vent area will be defined by two arches, front and back, with the rear one accepting the down-sloping firebricks from the cooking arch (its a semicircular arched cooking area like perk1018's but with external chimney). The chimney is an 8" x 12" flue pipe, 24" long with a spark suppressor/rain roof. Do you think that it would be too weak? I want to have arches spanning 19" at the rear and 24" at the front, behind a decorative arch.

[couchpotatoe]

GianniFocaccia
I should have read everything you said before replying...
The design of the arch was to have a very small rise using standard firebricks to allow for a more uniform opening height without resorting to steel. Wouldn't using solid firebrick be better than several smaller wedges that have to be mortared together?
Russ

[GianniFocaccia]

Wouldn't using solid firebrick be better than several smaller wedges that have to be mortared together?

From my limited understanding of vaulted structures, I believe an arch with a more conventional line of thrust is stronger than your design. I'm also not used to seeing an arch (or set of arches) that is only 2 1/2" thick. The strength provided by a mortared joint is more than adequate and there appears to be little gained by designing an arch with only two joints versus 10 or 11 (or more). An oven expands and contracts through its thermal cycles, and as I understand it, mortar plays an important part in absorbing and deflecting some of the heat so that all the stresses aren't placed directly on bricks, which can make them crack. Even in a shallower arch like yours, I think multiple bricks makes for a stronger arch.

John

[Tscarborough]

As a rule, the more segments (bricks) the better, not because it is an arch but because it is subjected to extreme heat cycling. Consider it engineered crack design.

[couchpotatoe]

Well, if figures there was a reason that my simpler arch idea was not being used. So, I will go with a traditional, but shallow, multi-segmented arch. Many thanks for the feedback. You probably saved me from some serious future problems.
Russ